1. Field of the Invention
The present invention is directed at skeletal plating systems, components thereof, and method of implant placement. These systems are used to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments during healing and fusion after surgical reconstruction of skeletal segments. Such systems may be comprised of bone distraction devices, skeletal plates, bone screws and/or bone cables, bone screw-to-plate locking mechanisms, and any additional instruments needed for implant placement.
2. Related Art
Whether for degenerative disease, traumatic disruption, infection or neoplastic invasion, surgical reconstructions of the bony skeleton are common procedures in current medical practice. Regardless of anatomical region or the specifics of the reconstructive procedure, many surgeons employ an implantable skeletal plate to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments during postoperative healing. These plates are generally attached to the bony elements using bone screws or similar fasteners and act to share the load and support the bone as osteosynthesis progresses.
Available plating systems used to fixate the cervical spine possess several shortcomings in both design and implantation protocols. These plates are manufactured and provided to the surgeon in a range of sizes that vary by a fixed amount. This mandates that a large number of different size plates must be made and inventoried—adding to cost for manufacturer, vendor, and end user (hospitals). More importantly, the pre-manufactured sizes may not precisely fit all patients forcing surgeons to choose between a size too small or too large.
Plates used to attach three or more vertebrae after removal of two or more discs are manufactured with an equal distance between screw holes in the vertical plane. For example, the distance between the pair of C3 (cervical bone #3) and the pair of C4 (cervical bone #4) screws is equal to the distance between the C4 and C5 (cervical bone #5) screws as well as the distance between the C5 and C6 (cervical bone #6) screws and so forth. This not only ignores the known anatomical difference in size between bones at different levels but also fails to anticipate that a patient's unique pathology may require more extensive bony resection at one or more levels, further adding to these differences. Thus, selection of a plate with a suitable total length may still produce improper fit at one or more levels.
Current cervical plates are not modular, and will not permit addition of one plate to another for extension of the bony fusion at a future date. It is accepted that fusion of a specific spinal level will increase the load on, and the rate of degeneration of, the spinal segments immediately above and below the fused level. As the number of spinal fusion operations have increased, so have the number of patients who require extension of their fusion to adjacent levels. Currently, the original plate must be removed and replaced with a longer plate in order to fixate the additional fusion segment. This surgical procedure necessitates re-dissection through the prior, scarred operative field and substantially increases the operative risk to the patient. Further, since mis-alignment of the original plate along the vertical axis of the spine is common, proper implantation of the replacement plate often requires that the new bone screws be placed in different bone holes. The empty holes that result may act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and lead to bone fracture and subsequent screw/plate migration.
Current plates may provide fixation that is too rigid. Since bone re-absorption at the bone/graft interface is the first phase of bone healing, fixation that is too rigid will not permit the bone fragments to settle and re-establish adequate contact after initial bone absorption. This process is known as “stress shielding” and will lead to separation of the bony fragments and significantly reduce the likelihood of bony fusion. Unsuccessful bone fusion may lead to construct failure and will frequently necessitate surgical revision with a second operative procedure.
Benzel (U.S. Pat. Nos. 5,681,312, and 5,713,900) and Foley (Pat. Applic. Pub. No. US2001/0047172A1) have independently proposed plating systems designed to accommodate bone settling. In either system, however, bony subsidence can be expected to cause one end of the plate to migrate towards an adjacent, normal disc space. This is highly undesirable since, with progressive subsidence, the plate may overlap the disc space immediately above or below the fused segments and un-necessarily limit movement across a normal disc space. Clearly, accommodation of bone settling at the plate's end is a sub-optimal solution.
Yuan et al described a multi-segmental plate consisting of two sliding parts in U.S. Pat. No. 5,616,142. While intended to be absorbable, this design may permit excessive play between the sliding segment and encourage bone screw loosening. In addition, this device does not permit application and maintenance of a compressive force across the bony construct. Baccelli noted these deficiencies in U.S. Pat. No. 6,306,136 and proposed a rigid plate capable of maintaining bony compression. However, the latter plate did not permit subsidence.
The implantation procedures of conventional plates in prior art practice have additional shortcomings. Distraction screws are used during disc removal and subsequent bone work and these screws are removed prior to bone plate placement. The empty bone holes created by removal of the distraction screws can interfere with proper placement of the bone screws used to anchor the plate and predispose to poor plate alignment along the long axis of the spine. This is especially problematic since the surgical steps that precede plate placement will distort the anatomical landmarks required to ensure proper plate alignment, leaving the surgeons with little guidance during plate implantation. For these reasons, bone plates are frequently placed “crooked” in the vertical plane and often predispose to improper bony alignment. Correct plate placement in the vertical plane is especially important in plates intended to accommodate bony subsidence, since the plate preferentially permits movement along its long axis. Thus, when the vertical axis of the plate and that of the spine are not properly aligned, the plate will further worsen the bony alignment as the vertebral bones subside.
The empty bone holes left by the removal of the distraction screws also act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and predispose them to bone fracture and subsequent screw/plate migration. Improper plate placement and bony fractures can significantly increase the likelihood of construct failure and lead to severe chronic pain, neurological injury, and the need for surgical revision with a second procedure.
In view of the proceeding, it would be desirable to design an improved bone plating system and placement protocol. The new device should provide ease of use, reliable bone fixation, adjustable length, modular design, and the ability to accommodate and control bone settling. The design should also maximize the likelihood of proper plate placement and avoid maneuvers that weaken the vertebral bodies. No current plating system addresses all of these concerns. Therefore, what is needed is a system and method that overcomes these significant problems found in the conventional systems as described above.